The transfer of small, dry toner particles, for example, toner particles of less than 5 microns in size from a photoconductor or other image member to a receiving sheet is extremely challenging. Studies on the forces which move small particles indicate that as the particle becomes smaller the effect of an electrostatic field is less on a particle compared to the effect of ordinary adhesive forces. This has made conventional transfer using an electrostatic field relatively ineffective in transferring such small particles. See, U.S. Pat. No. 5,084,735. Rimai et al, issued Jan. 28, 1992 and U.S. Pat. No. 4,737,433, Rimai et al.
U.S. Pat. No. 4,968,578, Light et al, issued Nov. 6, 1990; U.S. Pat. No. 4,927,727, Rimai et al, issued May 22, 1990; and U.S. Pat. No. 5,021,835, Johnson, issued Jun. 4, 1991, all describe a heat assisted toner image transfer method particularly usable with small particles. Two or more single color images are transferred in registration from an image member to a receiving sheet by heating the receiving sheet to an elevated temperature. The temperature of the receiving sheet is sufficiently high that the toner sticks to the receiving sheet and to itself. Preferably, the receiving sheet is heated from inside a transfer drum to which it is secured. The transfer drum and image member form a pressure nip with the combination of heat and pressure transferring the image. This method is particularly useful in transferring extremely small, dry toner particles, for example, toner particles having a mean particle diameter of less than 5 microns.
In a preferred form of the heat assisted transfer described in these references a receiving sheet having a heat-softenable outer layer is used. The receiving sheet is heated to a temperature which softens the outer layer and the first layer or layers of the toner images partially embed themselves in the heat-softened layer to assist in transfer of the first image or so. Further layers of toner from subsequent images or dense portions of the first image attach themselves to toner particles that are partially embedded. With extremely small, dry toner particles this method provides extremely efficient transfer with excellent resolution.
Although heat assisted transfer to a heat-softened layer provides the most efficient and highest resolution transfer of very small toner particles known in the prior art, it is not without problems. Depending somewhat on the materials, relatively high pressures are desirable, for example, pressures of up to 500 pounds per square inch and higher. Heating is accomplished generally through the receiving sheet. Even if the receiving sheet is carried on a metallic drum, it is somewhat difficult to maintain the temperature of the thermoplastic layer within limits that will sinter the toner without overheating the image member or blistering the receiving sheet. Overheating of the image member can cause damage to it, including a reduction of its ability to hold a charge. Overheating of the toner image can cause sticking to the image member and/or spreading of the image. It is known to provide a heating element inside a photoconductive drum which heats the drum to an elevated but safe temperature for the image member and thereby requires less heating from the transfer member. Even with this useful approach, temperature control at transfer is difficult with a receiving sheet receiving the images from a photoconductor.
An intermediate transfer member (sometimes herein called an "intermediate") has been used in both single color electrophotography and multicolor electrophotography. For example, U.S. Pat. No. 4,931,839, shows the use of an intermediate conductivity intermediate web to accumulate several single color toner images by separate electrostatic transfer from a photoconductive web. The multicolor image formed on the intermediate is electrostatically transferred to a receiving sheet and later fed to a separate fixing station. See also, U.S. Pat. Nos. 4,657,373; 4,068,937; 3,893,761; 4,453,820; and 4,542,978. In each of these references, the intermediate has a silicone rubber or other compliant surface which is used because of its affinity to toner at the first transfer step. At or before the second transfer step the image and, in some instances, the receiving sheet are preheated so that transfer and fusing can be accomplished in a single step. The intermediate is generally cooled before it returns to the original image member to pick up additional images for fear of damage to a photoconductor or other sensitive portion of the original image member.
U.S. Pat. No. 4,910,558 shows an intermediate drum which is internally heated and covered with compressible silicone rubber.
U.S. Pat. No. 4,912,514 shows an intermediate web with a conductive base and a fluoride coating with separate rapid heating components opposite the original transfer from a photoconductive drum and opposite a combination transfer-fusing position where the single image is transferred to and fused to a receiving sheet. The first transfer is said in the reference to involve fusing the toner on a photosensitive drum until it transfers to and is temporarily fixed on the surface of the intermediate.
U.S. Pat. No. 4,531,825 shows an intermediate roller having a heat conductive core with a silicone or fluoride resin coating. The original image member has a soft backing providing a larger nip for the first transfer.
U.S. Pat. No. 4,992,833 shows an intermediate sheet or web to which a single toner image is transferred by means not described. After the transfer the image is fused to the intermediate and kept warm until overlaid with a receiving sheet.
U.S. Pat. No. 5,110,702, issued May 5, 1992 (CIP of U.S. patent application Ser. No. 448,487, now abandoned) to Y. Ng discloses using thermally assisted transfer for three or four small particle color toner transfers to an intermediate.
Japanese Kokai 1-179181; published Jul. 17, 1989, shows a combination of heat and electric field used to transfer a toner image to a receiving sheet carried by either a drum or belt.